When a display device such as PDP, CRT and LCD is used, light from the outside is occasionally reflected at the surface of the display and difficulty arises in watching images on the display. In particular, as the size of flat panel displays increases recently, solving the above problem is becoming more important.
To solve the above problem, various antireflection treatments and anti-glare treatments have been made on various display devices. As one of such treatments, an antireflection film is used for various display devices.
The antireflection film has heretofore been prepared in accordance with a dry process such as vapor deposition and sputtering. A thin film of a substance having a low refractive index such as MgF is formed on a substrate film, or layers of a substance having a great refractive index such as ITO (indium oxide doped with tin) and TiO2 and layers of a substance having a small refractive index such as MgF and SiO are alternately laminated. However, the antireflection prepared in accordance with the dry process has a drawback in that the cost of production is high.
Recently, preparation of an antireflection film in accordance with a wet process, i.e., a coating process, has been attempted. However, the antireflection film prepared in accordance with the wet process has a drawback in that scratch resistance of the surface of the prepared film is inferior to that of the film prepared in accordance with the dry process.
The present inventors conducted intensive studies to overcome the above problems, and a film for optical applications comprising a hard coat layer which comprises a resin cured by an ionizing radiation and has a thickness in a range of 2 to 20 μm, a high refractivity layer which comprises a resin cured by an ionizing radiation and at least two metal oxides comprising an antimony-doped tin oxide (ATO) and has a refractive index in the range of 1.65 to 1.80 and a thickness in the range of 60 to 160 nm and a low refractivity layer which comprises a siloxane-based polymer and has a refractive index in the range of 1.37 to 1.47 and a thickness in the range of 80 to 180 nm, layers (A) to (C) being laminated at least on one face of a substrate film successively from the side of the substrate film, has been applied to the patent (Patent Reference 1).
In the film for optical applications having the structure described above, it is desired that the high refractivity layer has a great transmittance of light and exhibits excellent adhesion with the siloxane-based polymer in the low refractivity layer formed on the high refractivity layer. In the film for optical applications described in Patent Reference 1, the high refractivity layer exhibits excellent adhesion with the siloxane-based polymer since the layer comprises ATO. However, a problem arises in that the transmission of light decreases when the thickness of the high refractivity layer is increased while the excellent adhesion is maintained since ATO is in the form of spherical particles and, occasionally, the adhesion and the light transmission are not sufficiently satisfied simultaneously.
Antimony pentaoxide, which is a metal oxide, exhibits excellent adhesion with siloxane-based polymers. However, since antimony pentaoxide does not have a great refractivity, a problem arises in that the refractive index of the high refractivity layer does not reach the desired value although the adhesion is improved when antimony pentaoxide is comprised in a great amount.
In the film for optical applications having the above structure, it is advantageous that the low refractivity layer comprising the siloxane-based polymer comprises porous silica so that the refractive index is decreased. In this case, it is necessary for practical applications that an antifouling layer be formed on the low refractivity layer. Therefore, when the antifouling property can be provided to the low refractivity layer comprising porous silica and a siloxane-based polymer and the formation of the antifouling layer is not necessary, the efficiency of production can be improved, and the economic advantage can be obtained.
[Patent Reference 1]
Japanese Patent Application Laid-Open No. 2002-341103 Under the above circumstances, the present invention has an object of providing a film for optical applications which effectively prevents reflection of light on the surface of image display devices such as PDP, CRT and LCD, exhibits excellent scratch resistance and transmission of light, provides excellent adhesion between layers and can be produced at a low cost.